This invention relates to a high-speed differential protective arrangement for an electric power circuit and, more particularly, to a differential protective arrangement of this type for protecting said power circuit from faults internal to a predetermined zone of the power circuit.
The usual differential protective arrangement can discriminate between faults internal to a predetermined zone of the power circuit and those external thereto. But a problem that has been encountered when utilizing such a differential protective arrangement is that if a lightning arrestor is connected to the power circuit in said predetermined zone, the protective arrangement has difficulty discriminating between operation of the lightning arrestor and a true internal fault. More specifically, when such a lightning arrestor sparks over, initial spark-over is followed by follow-through current from the power circuit, and this follow-through current appears to the protective arrangement as internal fault current.
Despite this appearance, however, the protective arrangement should not operate in response to normal follow-through current since the lightning arrestor itself will interrupt such current within a short period, typically one-half cycle or less of power frequency current. One way to prevent the relay of the differential protective arrangement from operating in response to these conditions is to use a relatively slow relay, i.e., a relay which has a pick-up time sufficiently great so that it does not have time to pick up in response to lightning-arrestor current of the magnitude of follow-through current flowing for the normal duration of follow-through current.
A problem that has been present with this latter approach is that for lower magnitude fault currents, the relatively slow relay of the preceding paragraph requires an excessive time for pick-up. As a specific example, if the relay has a pick-up time of 1.5 cycles at 10 times pick-up voltage applied across input terminals to the relay, its pick-up time at 1.5 times pick-up voltage across these terminals will be about 3 cycles. Such long operating times cannot be tolerated in a high speed differential protective arrangement.